Request for quality of platings has become higher in recent years, and appropriate control of plating solutions has become much burden on the production sites. On the other hand, as an element of endeavor for cost-down coping with competition in price, automation of plating treatments has been progressing, and, as a result, an automatic control system for plating solutions has come to be indispensable.
Particularly, in the industry of plating in recent years, demand for electroless plating, particularly electroless nickel plating is very large, and such plating is used widely. As compared with electroplating, the electroless plating requires a very high frequency of analysis and an extremely high frequency of material replenishment, so that liquid control systems combining automatic analysis and automatic material replenishment have been developed and put to practical use over years. The liquid control systems have been widely adopted as an important element of electroless plating equipment.
Details of the above-mentioned systems are explained in literature such as “Automatic Control of Plating Bath”, Hyomen Gijutsu (Surface Technology), Vol. 34, No. 6, 1983, and “Automatic Control of Electroless Plating Bath”, Jitsumu Hyomen Gijutsu (Practical Surface Technology), Vol. 31, No. 10, 1984.
While a plating solution contains various components, the components analyzed by an automatic liquid control system are very limited components such as, for example, a component used as a standard for replenishment or a component most important for securing plating quality, and there is substantially no case where all components are analyzed.
Where there is a component which is not analyzed by the automatic liquid control system but must be periodically analyzed, manual analysis is carried out, and a control is conducted, if required. In practice, however, most components are substantially not analyzed or controlled.
In concrete, in the automatic liquid control system for electroless nickel plating solution, the components analyzed are usually Ni concentration and pH. Particularly, in electroless nickel plating, control of the Ni concentration is the most important. Since the Ni concentration is gradually lowered due to consumption of the Ni component when electroless nickel plating is carried out, the Ni component is sequentially replenished for maintaining the Ni concentration at a predetermined value. It is general liquid control means to replenish other components also, in proportional manner, with the amount of the Ni component replenished as a measure. In other words, the Ni concentration is utilized as a standard for ideal control of all the components, so that the accuracy of analysis of the Ni concentration in the liquid control is very important.
As an analyzing method for Ni concentration, chelatometric titration and absorptiometry are general ones, and, at present, the absorptiometry is generally used in an automatic liquid control system for the electroless nickel plating. The absorptiometry has a very long history as one means of analyzing composition by instrumental analysis, and includes various techniques from colorimetry in which concentration is measured through comparison of solution colors to spectrophotometry in which absorbance is measured by use of light with a wavelength in an extremely narrow range close to monochromatic light. The principles and analyzing techniques of absorptiometry are described in detail in “Instrumental Analysis Guide Book” (edited by the corporate juridical person the Analytical Chemical Society of Japan, published by Maruzen Co., Ltd. Jul. 10, 1996), and “Experiments and Computation in Quantitative Analysis” (written by Seiji Takagi, published by Kyoritsu Syuppan Co., Ltd., first published on Nov. 5, 1961). In actual quantitative analysis of the Ni concentration in an electroless nickel plating solution by absorptiometry, absorbency of light with a wavelength in green color portion in the visible region is measured.
The electroless nickel plating solution contains various complexing agents, the Ni component is present as an Ni complex ion, which strongly absorbs light in the wavelength region of green color, and there is a good proportionality relationship between the absorbance in the wavelength region and the Ni concentration. By utilizing this characteristic feature, quantitative analysis with high accuracy is performed. To perform measurement in a specified wavelength region, the light must be spectrometrically conditioned, so that most systems for analysis adopt the technique of selecting light by interference filter. Alternatively, there is a method in which a wavelength extremely close to monochromatic light is obtained by monochrometer using a diffraction grating or a prism. However, this method is rarely used because of complicated mechanism and comparatively high cost, and because such a high spectrometric treatment is not needed for analytical accuracy of Ni concentration required in the conventional liquid control systems.
Not limited to electroless plating, there are many cases where the absorptiometry is used as an automatic liquid control system or liquid analysis method, and many patent applications are found on patent investigations.
However, there is found almost no proposal as to the measuring method in an automatic liquid control system for an electroless composite plating solution.
As mentioned above, although automatic liquid control systems for electroless plating solution have been put to practical use and widely spread, use of the existing liquid control system for the purpose of controlling an electroless composite plating leads to various problems. First, in the case of an electroless nickel plating solution, many of the existing systems use absorptiometry as the method of measuring Ni concentration. In that case, the wavelength of the light for measurement is the wavelength at which the absorption of light due to Ni complex is present. In many cases, the measurement is conducted at one wavelength in the visible region (VIS; wavelength range from 400 to 750 nm).
In the case of measuring a composite plating solution, however, the incident light is not only transmitted straight and absorbed but also reflected, diffracted or scattered by the suspended particles. The light reflected, diffracted or scattered by the suspended particles leads to apparent decrease of the transmitted light, and cannot be distinguished from the decrease of the transmitted light due to absorption by the objective component, resulting in that the amount of the objective component is erroneously judged to be more than the real amount. In addition, the degree of influence of the suspended particles varies depending on the kind, particle size distribution and concentration of the suspended particles, and depending on various factors of the plating solution. For example, when the plating solution is specified, the influence of the suspended particles is comparatively stabilized, so that the concentration of the objective component can be measured with comparatively good accuracy by preliminarily deeming a fixed value as the decrease of transmissivity due to turbidity. However, the electroless plating solution shows a large variation in composition as it is used, and influences of the variation must be corrected, so that the method of allowing for the influence of turbidity by use of a fixed value is limited in practicality.
In addition, when a special trouble is generated, for example, when special foreign particulates are taken into the plating solution and bad dispersion is generated, the turbidity is greatly changed, resulting in large errors in the analytical results of the objective component. Besides, also when a plating solution sampling mechanism is out of order so that a plating solution with uniform dispersion of particulates cannot be sampled, there is a fear that gravy and fatal analytical errors would be generated.
Thus, it can be said that it is substantially impossible to secure the required accuracy and reliability by simply using the analytical method in the conventional systems. Although there are some countermeasures against the problems in the analysis of an electroless composite plating solution, the countermeasures have respective drawbacks.
For example, the method of measuring after separating the particulates dispersed in the plating solution by filtration, sedimentation, centrifugal separation or the like is accompanied by difficulties or cost demerit as to the mechanism for continuously or intermittently performing the separation, and liquid conditioning is very difficult since the plating solution is wasted attendant on the analysis. On the other hand, the method of analyzing by chelatometric titration is attended by high complicatedness of system, and requires a sampling device with very high precision and reliability for securing accuracy. In addition, a large amount of waste liquid is generated by analysis, and there is need for expendable chemicals for analysis such as an indicator and a titration liquid; thus, the chelatometric titration method has many minus factors, as compared with the absorptiometry.
It can be said that an ideal method is to perform measurement while keeping the plating solution as it is and return the plating solution into the plating tank in a recirculating cycle manner, without processing the plating solution for analysis or wasting the plating solution as in the general automatic analysis and control system for electroless plating solution according to the prior art.